1. Field of the Invention
The invention is directed to a method for avoiding image occlusions in the production of images using a nuclear magnetic resonance tomography apparatus are operated with multi-echo sequences of the type wherein a cross-magnetization of spins is generated in an examination subject at a time to with an excitation radio-frequency pulse, this excitation radio-frequency pulse being followed at times t.sub.1, t.sub.3, t.sub.5, . . . by at least two refocusing radio-frequency pulses that re-phase the cross-magnetization, and read-out intervals following at times t.sub.2, t.sub.4, t.sub.6, . . .
2. Description of the Prior Art
Imaging sequences known as multi-echo sequences that have the following features in common. A first radio-frequency pulse (excitation radio-frequency pulse) generates a cross-magnetization. At least two further radio-frequency pulses (radio-frequency refocusing pulses) that follow the radio-frequency excitation pulse re-phase this cross-magnetization and thus generate measurable MR signals triggered by the first pulse. For example, these MR signals can be phase-encoded in the same way, so that the relaxation of the nuclear magnetic resonance signal can be observed or the signal-to-noise ratio can be approved by averaging. It is more standard in modern applications, however, to select a different phase-encoding of the MR signals generated following the excitation radio-frequency pulse, so that the data acquisition for the reconstruction of a MR image becomes faster. Such methods are referred to as turbo-spin echo sequences. A measuring time that is shortened further is achieved when, following every refocusing radio-frequency pulse, the read-out gradient is also multiply reversed, and thus a plurality of signals are respectively acquired. Such a pulse sequence as disclosed in U.S. Pat. No. 5,270,654 is referred to as a gradient spin echo sequence.
It is known that a switched, linear magnetic field gradient as inherently required for the MR imaging cannot be generated isolated in a basic field. On the contrary, switched magnetic field gradients are always connected to transverse field components as a result of the Maxwell equations. This problem is discussed in a number of references.
D. G. Norris "Phase Errors in NMR Images", SMRM Abstracts 1985, pp. 1037-1038 points out the problem of phase distortions due to an undesirable gradient component in conjunction with the traditional spin-warp technique. For solving this problem, it is proposed that one bipolar pulse be replaced by two mono-polar pulses separated by a 180.degree. radio-frequency pulse.
The problem that has been presented is primarily discussed in conjunction with echo planar imaging (EPI) in the literature. This stems from the fact that the undesired effect becomes more disturbing as the gradient amplitude becomes higher in relationship to the basic magnetic field. Especially short, and thus high, gradients are required, however, in the EPI method. The following solutions have thereby been proposed.
R. Coxon and P. Mansfield "EPI Spatial Distortion in Non-Transverse Planes", SMRM Abstracts 1989, p. 361 propose that the spatial distortion caused by undesired gradient components be eliminated by after-processing in the acquired data sets or by dynamic adjustment of shim currents.
D. G. Norris and J. Hutchinson "Concomitant Magnetic Field Gradients and Their Effects on Imaging at Low Magnetic Field Strength", Magnetic Resonance Imaging, Vol. 8, pp. 33-37, 1990, propose for spin echoes, the use of bipolar pulses as applied, for example, for refocusing given flux, be replaced by using unipolar pulses. These unipolar pulses are separated by a 180.degree. radio-frequency pulse, so that they have the effect of a bipolar pulse.
R. M. Weisskoff et al. "Nonaxial Whole-Body Instant Imaging" MRM 29, pp. 796-803 (1993) suggest inserting a pre-phasing gradient between a 90.degree. pulse and a 180.degree. for the reduction of phase errors in a EPI pulse sequence.
None of these references, however, is concerned with multi-echo imaging. It has been found that occlusions occur in the image given multi-echo sequences, particularly when the gradient field strength is high in comparison to the basic field strength. The problem thus becomes especially serious given low basic field strengths and/or strong gradients.